1. Field of the Invention
The present invention relates to a toner for developing a latent electrostatic image, which contains a vinyl resin attached onto a surface of the toner, and can be used in electrophotography, and to a method for producing the toner.
2. Description of the Related Art
As a toner used for an electrophotographic image forming method, a toner using polyester having excellent fixability is desired. Moreover, in order to obtain a fine image, a toner having a substantially spherical shape, and narrow particle size distribution, specifically, several micrometers, is desired. As a method for obtaining such toner (colored particles), the following methods are known: a dissolution suspension method, in which a binder resin such as a polyester resin, a colorant and a releasing agent are dissolved or dispersed in a solvent, and the dispersion liquid is dispersed in an aqueous medium so as to obtain colored particles; and an emulsification association method, in which fine particles of a polyester resin, a pigment and a releasing agent, etc. are aggregated in the presence of an aggregated salt, so as to adjust shapes of particles, to thereby obtain colored particles.
However, these toners, which are produced in an aqueous medium and contain a polyester resin as a main component, are likely to have a low chargeability, and it is difficult to use these toners in an electrophotographic process. Particularly, in one component developing process having less opportunity to charge a toner, since the toner is less charged, background smear, toner falling, or the like outstandingly occurs. Thus, demand has arisen for improvement of toner chargeability.
As one of the methods for improving the toner chargeability, a method of allowing a styrene-acrylic resin having excellent chargeability to exist on a toner surface has been known (see Japanese Patent Application Laid-Open (JP-A) No. 2006-285188).
However, in this method, since the polyester resin is covered with the styrene-acrylic resin, the polyester resin cannot exhibit excellent fixability, it inherently has.
In JP-A No. 2007-233169, disclosed is a toner for developing a latent electrostatic image, which satisfies both low temperature fixability and heat resistant storage stability, has excellent offset resistance, a controllable structure, and excellent charge amount without contamination of a developing device and other members/devices. Particularly, it discloses a toner having a core part which contains a binder resin having a polyester skeleton, a colorant, and a releasing agent, and a vinyl copolymer resin part, for the purpose of providing a non-magnetic toner for electrostatic charge development and a method for producing the toner, a developer using the toner, a toner container and an image forming apparatus.
The invention disclosed in JP-A No. 2007-233169 is similar to the present invention, in that the toner contains a core part containing a binder resin having a polyester skeleton, a colorant, and a releasing agent, and a shell part formed of a vinyl copolymer resin. However, in JP-A No. 2007-233169, the conventional problem that the toner cannot exhibit excellent fixability of the polyester resin has not been solved.
The toner disclosed in JP-A No. 2007-279689 contains a core part which contains a binder resin having a polyester skeleton, a colorant, and a releasing agent, and a shell part formed of a crystalline polyester resin having high polarity. In the case where the toner has such a structure, the toner has excellent fixability, but insufficient chargeability since the crystalline polyester resin having poor chargeability is located on the toner surface, and sufficient printing quality cannot be achieved.
The toner disclosed in JP-A No. 2008-268353 is a toner containing a crystalline polyester resin, and having a surface layer, which contains urethane, polyester, a styrene-acrylic resin and a crystalline polyester resin. However, even though such surface layer is provided on the toner, toner chargeability is not sufficient, and sufficient printing quality cannot be achieved.